Reactor ScaleUp

The other disadvantage of this type of system is that the temperature in the condenser is lower than in the reactor because of both the lower pressure and the more volatile components that are in the vapor phase. This means that a lower-temperature coolant must be used in the condenser compared to what could be used in a jacket- or coil-cooled system. 

One of the most challenging aspects of chemical engineering is the problem of scaling up a process unit from a small laboratory or pilot plant to a large commercial size. Reactors are perhaps one of the more difficult to deal with. In this section we show quantitatively what the heat transfer scaleup problem is for a CSTR. 

Suppose that a pilot plant reactor has a volume of 0.019 m3 [5 gal (gallons)] and is fed 3.506 g/s of feed with a density of 801 kg/m3 and a temperature of 294 K. The reaction is A — B, which takes place in the liquid phase at a reactor temperature of 333 K. The concentration of reactant in the feed is 8.01 kmol/m3 and the specific reaction at 333 K is 2.409 x 10-4 s 1. With the given feedflow, reactor volume, and temperature, the reactant concentration of the product stream is 3.926 kmol/m3 (conversion is 51%). 

The reactor is cooled by a circulating cooling water system. The heat of reaction is 69.71 x 106 J/kmol. The heat that must be transferred to the jacket is 817 W. If an aspect ratio (height to diameter) of 2 is assumed, the diameter and jacket area can be calculated  

Assuming an overall heat transfer coefficient of 851 W K-1 m-2, the required temperature differential between the reactor and the jacket is only 2.9 K, giving a jacket temperature of 330 K. If the supply cooling water temperature is 294 K, the cooling water makeup flowrate is 5.43 g/s. 

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For a new process, the basis used for reactor scaleup should be discussed with the licensor. Scaleup of other equipment may also require discussions. 

The licensor s basis for sizing has already been discussed and agreed to or changed. For an olefin plant, the number of steam crackers of the licensor s standard size is firm. For a new process, reactor scaleup methods have been agreed to. For a coal gasification plant, gasifier size. 

Thermal effects can be the key concern in reactor scaleup. The generation of heat is proportional to the volume of the reactor. Note the factor of V in Equation (5.32). For a scaleup that maintains geometric similarity, the surface area increases only as Sooner or later, temperature can no longer be controlled,... 

It may not be feasible to have an adequately low value for Q>AtlP and still scale using geometric similarity. Recall that reactor scaleups are done at constant t. The problem is that the pilot reactor would require too high a flow rate and consume too much material when is small enough (i.e., R is... 

Tarmy, B. L., Chang, M., Coulaloglou, C. A., and Ponzi, P. R., The Three-Phase Hydrodynamic Characteristics of the EDS Liquefaction Reactors Their Development and use in Reactor Scaleup, Proc. 8th Int. Symp. Chem. Reaction Eng., 30 239 (1984)... 

Not until the industrial era did people want to make large quantities of products to sell, and only then did the economies of scale create the need for mass production. Not until the twentieth century was continuous processing practiced on a large scale. The first practical considerations of reactor scaleup originated in England and Germany, where the first large-scale chemical plants were constmcted and operated, but these were done in a trial-and-error fashion that today would be unacceptable. 

The final subject discussed in this chapter is the issue of reactor scaleup. Moving from a laboratory test tube in a constant temperature bath to a 20-L pilot plant reactor to a 200,000-L commercial plant reactor involves critical design and control decisions. One major problem is the reduction of the heat transfer area relative to the reactor volume (and heat transfer duty) as we move to larger reactors. This has an important effect on temperature control and reactor stability. 

Trambouze, P., Chem. Eng. Progress, Feb. 1990, 23-31 Reactor Scaleup Methodology... 

The flow regime plays a very important role in reactor scaleup. If the data obtained in the pilot-scale reactor are to be useful for a larger-scale reactor, the flow regime in these two reactors must be the same. The flow regimes in a variety of fixed-bed operations are described in Chaps. 6 to 8. 

Since gas causes most of the mixing in three-phase reactors, its distribution is, of course, very important. For small-diameter columns, the nature of a gas distributor is known to have a significant effect on RTD. Similar information on large-diameter columns is presently unavailable. For reactor scaleup purposes, such information is desirable. Studies should consider various reactor configurations and flow regimes. 

Schematic diagrams of various types of fixed-bed reactors are shown in Fig. 1 -1. The reactor thus reproduces the industrial reactors on a small scale and it can be used to obtain the required data for reactor scaleup.

Use of S without subscripts indicates that Equation 1.58 is satisfied. This is true for the great majority of reactor scaleups for both liquids and gases. 

Constraining scaleup factors to have a constant value for the mean residence time is appropriate to reactor scaleups. The general scaleup factors are then subject to the constraints that Sthroughput = 5 and S = S Sr. Imposing a constant value for t means that there are only two independent variables. We treat 5 as one of them. Then either... 

Thermal effects are often the key concern in reactor scaleup. The generation of heat is proportional to the volume of the reactor. Note the factor of V in Equation 5.31. For a scaleup that maintains geomedic similarity, the surface area increases only as Sooner or later, temperature can no longer be controlled by external heat transfer, and the reactor will approach adiabatic operation. There are relatively few reactions where the full adiabatic temperature change can be tolerated. Endothermic reactions will have poor yields. Exothermic reactions will have thermal runaways giving undesired byproducts. It is the reactor designer s job to avoid limitations of scale or at least to understand them so that a desired product will result. There are many options. The best process and the best equipment at the laboratory scale are rarely the best for scaleup. Put another way, a process that is less than perfect at a small scale may be better for scaleup precisely because it is scaleable. 

The concentration of hexadecane falls much more rapidly than the number of moles of reactant. If the change in total moles is not allowed for, it can lead to errors in determination of reaction order and in reactor scaleup. 

F-T mathematical model and reactor scaleup Texaco s proprietary F-T model was successfully used to fit the experimental data from Rentech bench unit which has a diameter of 3.8 cm (1.5-inch). It was also successfully... 

Results of simulations. Comprehensive computations have been carried out to detect the behavior of the FT slurry bubble column reactors ( 15, 85). Owing to space limitations only few results will be presented here which are related to reactor scaleup. 

Fig. 9.29. Typical reacting CFD simulation results and performance evaluation for the coaxial jet tubular LDPE reactor (scaleup factor for diameter is 1 50).

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